The present invention relates generally to diagnostic imaging and, more particularly, to a collimator assembly having collimating components independent of shielding components.
Typically, in computed tomography (CT) imaging systems, an x-ray source emits a fan-shaped beam toward a subject or object, such as a patient or a piece of luggage, along a projection path. Hereinafter, the terms “subject” and “object” shall include anything capable of being imaged, such as a medical patient. The beam, after being attenuated by the subject, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is typically dependent upon the attenuation of the x-ray beam by the subject. Each detector element of the detector array produces a separate electrical signal indicative of the attenuated beam received by each detector element. The electrical signals are transmitted to a data processing system for analysis which ultimately produces an image.
Generally, the x-ray source and the detector array are rotated within the gantry about an imaging plane and around the subject. X-ray sources typically include x-ray tubes, which emit the x-ray beam at a focal point. X-ray detectors typically include a collimator for collimating x-ray beams received at the detector as well as reducing x-ray scatter, a scintillator for converting x-rays to light energy, a plurality of reflector elements disposed between the scintillators to reduce cross-talk emissions, and photodiodes for detecting the light output of the scintillators and producing electrical signals therefrom.
As stated above, typical x-ray detectors include a collimator assembly for collimating x-ray beams such that x-ray scattered by the patient and detected by the detector cells is minimized. Reducing this acceptance of scattered x-rays reduces image noise thereby improving the final reconstructed image. The collimator assembly is customarily a single structure defined by a plurality of plates or walls that extend along one or two dimensions above the scintillator array. Generally, the collimator plates have a width or thickness orthogonal to the projection path that is substantially similar to the width of the reflector material disposed between each of the scintillators. As such, it is paramount for the manufacturing and assembly processes to precisely align each of the plates of the collimator assembly with the reflector material gaps between scintillators thereby reducing scattering without blocking or minimizing the blocking of any of the active area or scintillator areas of the cells.
Known manufacturing processes attempt this exact alignment by constructing a continuous collimator assembly that is sized to dimensionally match the width and length of the entire detector array. That is, the scintillators are arranged in an array or pack and positioned on a tooling base such that the array or pack is fastened to the continuous collimator assembly. As such, the plates of the continuous collimator assembly must exactly align with the reflector walls or elements between each of the pixilated scintillator cells; otherwise, the collimator assembly must be discarded and a new collimator manufactured or the scintillator arrays or packs must be discarded and a new pack or array manufactured. This process requires excessively tight tolerancing and requires great operator skill and patience to assemble. Accordingly, these known processes are susceptible to waste of parts, material, and labor.
In addition to reducing x-ray scattering, known collimator assemblies also perform a shielding function. That is, the collimator assembly typically comprises relatively large amounts of tungsten that operate to reduce x-ray scattering. However, the amount of tungsten generally used exceeds that which is minimally required. The additional tungsten is used to increase the cross-sectional width of the collimator plates. The additional width is needed as a shield for various portions of the scintillator, reflector and photodiode. For example, it is not unusual for scintillator edges to be degraded during the manufacturing process. Further, it is common for exposed portions of reflector to discolor when exposed to x-rays or Î3-rays which negatively affect the reflectivity of the reflector. Furthermore, x-rays, without shielding, could penetrate the reflector and be absorbed by the photodiode array resulting in unwanted noise signals. Accordingly, the width of the collimator is generally increased with additional tungsten and other materials such that the degraded edges and otherwise-exposed reflector and the photodiode are “shielded” from direct x-ray impingement. Additionally, tungsten absorbs x-rays or Î3-rays and, as such, radiation dosage to the patient must be sufficient to accommodate the absorption characteristics of the tungsten. This standard construction is shown in FIG. 8.
Referring now to FIG. 8, a cross-sectional schematic diagram of a standard detector is shown. The detector 2 has a plurality of scintillators 3 arranged in an array 4 that are designed to output light upon the reception of high frequency electromagnetic energy such as x-rays or Î3-rays whereupon the light is detected by a photodiode array 5. The scintillators 3 are individually defined by a series of reflector elements or walls 6 that are connected by a reflector bridge or layer 7. The reflector elements 6 reduce cross-talk between adjacent scintillators 3. In front of or secured to the reflector layer 7 is a plurality of collimator plates or elements 8 that collectively form a collimator assembly 9. The collimator assembly 9 collimates radiation projected toward the scintillator array, reduces x-ray scatter and shields scintillator edges, otherwise-exposed reflector portions and shields the photodiodes from x-rays that penetrate the reflector layer 6. The width of each single piece collimator element 8 has a width wc that is substantially equal to or slightly wider than the width wr of each reflector element 6. As stated above, this rough or approximate equality of widths of the collimator elements and the reflector elements requires precise placement of the collimator assembly 9 to the scintillator array 4.
Additionally, as noted above, to sufficiently shield the scintillator edges, reflectors and photodiodes, the width of the collimator plates must be wider than is otherwise necessary for collimation and x-ray scatter reduction. Further compounding this size limitation is that a specified aspect ratio must be maintained. One skilled in the art will readily appreciate that “aspect ratio” is the dimensional ratio of the length or height of the collimator plates in the y-direction relative to the width between the collimator plates in the x-direction. As such, to maintain proper collimation, the height of each collimator plate must be made much larger than is needed to shield the scintillator edges, reflectors and photodiodes. All of which prolongs the manufacturing process, increases associated costs, and increases the amount of radiation necessary for data acquisition.
Therefore, it would be desirable to design a collimator assembly wherein collimating components are separated from shielding components. It would be further desirable to manufacture such a collimator assembly wherein the collimating components are thinner than the shielding components thereby improving manufacturing tolerances, reducing material needs, and reducing dosage requirements needed for ran imaging session.